Apologies for resurrecting an old thread, but while both of the below methods worked when I first started having this issue, now nothing mentioned here works.
Ie. regardless of what I try and do, I cannot make Lightroom maintain the same crop aspect between different images...
Originally Posted by lynnb
In the Develop module, select Command ->Copy (on Mac). The Copy Settings dialog box will appear, with check boxes for all development settings including Straighten Angle and Crop Aspect Ratio. Select the check boxes for the development settings you want to copy, select other photo/s and Paste.
Even easier and quicker: Develop the first photo, including Crop. Then with that picture selected, extend your selection to all photos, and click the Sync... button on the bottom right hand side of the screen. The Sync dialog box allows you to select which development settings are applied to the selection.